When a knock-out punch actuated by a hydraulically driven ram pierces through sheet metal being punched, the pump continues to supply hydraulic fluid to the ram despite the punch having completed its task and having no resistance in front of it except the die. The result is possible damage to the punch by its bottoming out against the die. Prior patents which generally relate to systems that sense a sudden drop in system fluid pressure and to thereupon drastically reduce the outlet unit pressure of the pressurized fluid actuating a tool or a punch piston include the following:
1. U.S. Pat. No. 4,099,436 issued July 11, 1978, describes a punch drive control system employing an air pressure switch to sense a sudden drop in pressure upon the punch's piercing the sheet material and to actuate air valves to bias the first punch piston as well as a second piston to move in the reverse direction away from the sheet material. The system is said to eliminate noise and vibration normally occurring when the punch just breaks through the sheet material.
2. U.S. Pat. No. 3,138,174 issued June 223, 1964, describes a fluid flow valve substantially reducing a flow of fluid therefrom when downstream pressure is drastically reduced. The valve includes only a single spring biased valve operable on differential inlet/outlet pressure to shut off fluid flow if the outlet pressure is drastically reduced relative to inlet pressure.
3. U.S. Pat. No. 2,478,210 issued Aug. 9, 1949, describes an automatic shut-off valve for interrupting supply pressure between an inlet port and an outlet port.
4. U.S Pat. No. 1,531,406 issued Mar. 31, 1925, illustrates a governor for fluid-pressure-actuated tools to control over-speeding or racing of the tool when it is disengaged from a workpart by automatically reducing the supply of air to the tool.
5. U.S. Pat. No. 4,543,976 issued Oct. 1, 1985, describes a control valve for a vehicle hydraulic system. The valve arrangement shown is used to control or limit charging pressure of a hydraulic accumulator by a high pressure pump and to recharge when accumulator pressure drops to a so-called "cut-in" pressure as a result of use of the vehicle hydraulic system, for example, power steering. A sudden drop in line pressure of the system would cause recharging of the accumulator by the pump.
None of the above inventions sets forth a system that can overcome the problems encountered by operators of hydraulic knock-out punch systems.